Autopilot having an adaptive deadband feature

ABSTRACT

A marine autopilot having a deadband feature to reduce ineffectual rudder movement and unnecessary wear on the rudder and associated drive apparatus. In a first deadband operation, four deadband values are computed based on various parameters in order to provide optimum system performance and circuit redundancy. The deadband values are a function of the maximum allowable heading error, the maximum allowable angular velocity of the vessel, and the vessel speed. A second deadband operation compensates for noise introduced into the autopilot system by the speed sensor, compass, and roll sensor.

This application is a continuation of application Ser. No. 07/794,757filed Nov. 19, 1991, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to autopilots and more particularly toa marine autopilot having an adaptive deadband feature.

As is known in the art, marine autopilots are used to maintain a ship,or vessel on a fixed course while the vessel encounters environmentalvariations such as changes in wind speed and direction and changes insea conditions. Preferably, the vessel course is maintained with minimumintervention by the operator of the vessel. In particular, the autopilotadjusts the position of the vessel's rudder in order to compensate forcourse deviations caused by changes in, inter alia, waves, wind,currents, and vessel speed.

Some marine autopilots use a proportional plus integral plus derivative(PID) control law to maintain the vessel on a desired course (i.e.during course keeping operation) and a proportional plus derivative (PD)control law to change the course of the vessel (i.e. during coursechange operation). Such an autopilot provides an output signal,hereinafter referred to as a rudder control signal, which corresponds toa desired change in the position of the rudder. During course keepingoperation, the rudder control signal is proportional to the summation ofthe following terms: an error signal (i.e. the difference between adesired course and the actual instantaneous vessel heading), the timeintegral of the error signal, and the time rate of change of the errorsignal. Whereas, during course change operation, the rudder controlsignal is proportional to the summation of the error signal and the timerate of change of the error signal.

More particularly, each term of the conventional PID and PD control lawshas a gain value associated therewith. The gain value associated withthe error signal may be referred to generally as a proportional gainvalue, the gain value associated with the time integral of the errorsignal may be referred to generally as a trim value, and that associatedwith the time rate of change, or derivative of the error signal may bereferred to generally as a counter rudder value. Thus, during coursekeeping operation for example, the rudder control signal (i.e. thecourse keeping signal) is equivalent to K_(p) e(t)+K_(d) e(t)+K_(i)∫e(t)dt, where e(t) is the error signal K_(p) is the proportional gainvalue K_(i) is the trim value, and K_(d) is the counter rudder value.

In such a control system, the proportional term (i.e. K_(p) e(t))provides rudder movement proportional to the error signal. Thederivative term (i.e. K_(d) e(t)) provides damping in the sense thatonce the vessel yaws, the derivative term provides resistance to suchmotion, or angular velocity. In this way, the derivative term reducesovershoot of the vessel past a desired course. The integral term (i.e.K_(i) ∫e(t)dt) provides compensation for low frequency disturbances,such as wind, by providing a bias on the rudder position to offset theeffect of such disturbances. Generally, the rudder control signalprovided during course change operation (i.e. course change signal) isas described above with the exception that the integral term (i.e. K_(i)∫e(t)dt) is nulled, or excluded, thus resulting in proportional plusderivative control.

As is also known in the art, autopilots often include manual adjustmentcapabilities for modifying various control system parameters. Forexample, autopilots often include a manually adjustable deadband featurewhereby the rudder motor is actuated only in response to rudder controlsignals having a value greater than an operator adjustable threshold, ordeadband value. Generally, the operator or helmsman is instructed todecrease the deadband value when the vessel is heading in the directionof the waves and to increase such value in following seas (i.e. whenheading away from the waves). In this way, high frequency perturbationsin the vessel's heading, generally occurring when the vessel is headingin the direction of the waves, will not be compensated. Increasing thedeadband value when heading into the waves is desirable since, in suchconditions, movement of the rudder to compensate for high frequencyheading perturbations will have little effect on maintaining the desiredvessel course. In other words, a certain amount of yawing of the vessel,as it heads into the waves, is unavoidable. By allowing the highfrequency perturbations in the vessel's heading to go uncompensated,ineffectual rudder movement is avoided. In this way, unnecessary wear onthe rudder and associated drive apparatus is minimized.

However, manual adjustments somewhat defeat a primary purpose of anautopilot; namely, to maintain a fixed vessel course with minimaloperator intervention. Further, such adjustments are not trivial and maybe difficult for one unskilled or inexperienced in boating and/orautopilot operation. Thus, although the basis for providing a manualdeadband adjustment is to improve the efficiency of rudder operation byeliminating ineffective rudder motion, rudder efficiency of an autopilothaving a manually adjustable deadband feature may actually be degradedby incorrect or excessive adjustments of a manually adjustable deadbandfeature.

One technique known in the art for minimizing operator intervention withregard to manually adjustable features is to provide a fixed number ofsettings for the manual adjustments. In other words, a fixed number ofpossible deadband settings may be provided for selection by the operatorof the vessel. However, the performance of such autopilots may bedegraded by the lack of fine tuning capability for such adjustments.

Another technique known in the art for minimizing operator interventionwith a deadband feature is to adjust the deadband value as a function ofthe number of reversals in the rudder's direction occurring during afixed period of time. In this way, such an arrangement raises or lowersthe deadband value as a function of rudder activity regardless of coursekeeping performance.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of the presentinvention to provide a marine autopilot having an improved deadbandfeature.

Another object is to provide a marine autopilot with an adaptivedeadband feature.

A still further object of the present invention is to provide a methodfor adaptively, or automatically computing a deadband value.

A further object is to compute automatically a deadband value as afunction of the deviation of the vessel from a desired course.

Another object is to compute automatically a deadband value as afunction of the angular velocity of the vessel.

Yet another object of the present invention is to provide compensationfor low frequency disturbances, such as wind, when the rudder ordersignal has a value less than an automatically determined deadband value.

These and other objects of the present invention are attained generallyby providing an autopilot adapted for use on a vessel having a steeringmechanism, a source of a signal corresponding to the actual speed of thevessel, means for providing a velocity scheduling signal in response tosaid actual speed signal, and a source of a signal corresponding to theactual heading of the vessel. The autopilot includes an operatoractuable controller for providing a signal corresponding to a desiredcourse for the vessel and means responsive to the desired course signaland the actual heading signal for providing an error signalcorresponding to the deviation between the desired course and the actualheading. The autopilot further includes means responsive to the errorsignal for generating an intermediate rudder control signal, a deadbandthreshold signal generator, and an actuator for actuating the steeringmechanism in response to the intermediate rudder control signal when theintermediate rudder control signal exceeds the deadband thresholdsignal. The deadband threshold signal generator includes meansresponsive to the velocity scheduling signal for automatically adjustingthe value of the deadband threshold signal.

With this arrangement, the deadband threshold signal is automaticallyadjusted. More particularly, the deadband threshold signal is increasedwhen the vessel is heading into the direction of the waves and isdecreased in following seas. This feature is desirable since, whenheading into the waves, disturbances in the vessel's heading generallyoccur at a relatively high frequency. Since the rudder is unable toeffectively respond to such high frequency disturbances and since anattempt to do so causes unnecessary wear on the rudder and associateddrive apparatus, such disturbances are ignored. In this way, ineffectualrudder movement is reduced in order to concomitantly reduce wear on therudder and associated drive apparatus.

In accordance with a further aspect of the present invention, thedeadband threshold signal generator comprises means for generating afirst predetermined value in response to the error signal, meansresponsive to the error signal for differentiating such signal, meansfor generating a second predetermined value in response to thedifferentiated error signal, and means for selecting the greater of thefirst and second predetermined values to provide the deadband thresholdsignal. The deadband threshold signal generator may also include meansresponsive to the velocity scheduling signal for determining a thresholdlimit value and means for selecting the greater of the firstpredetermined value, the second predetermined value, and the thresholdlimit value to provide the deadband threshold signal.

With this arrangement, the deadband threshold signal, to which theintermediate rudder control signal is compared, is a function of variousparameters. For example, a first deadband value is computed as afunction of the deviation between the desired course and the actualheading. Another deadband value is generated in response to the timerate of change of the error signal and corresponds to a maximumallowable angular velocity for the vessel. Furthermore, a third deadbandvalue is a function of the velocity scheduling signal and represents adeadband limit value. The greater of the deadband values thus generatedis selected for providing the deadband threshold signal to which theintermediate rudder control signal is compared. Such an arrangementprovides fault protection in the event that a sensor used to provide oneof the predetermined values fails. For example, if the sensor providingthe actual heading signal were to fail, the deadband threshold signalwould still be provided by another one of the predetermined values.

In accordance with a further aspect of the present invention, a marineautopilot for use on a vessel having a rudder and a source of a signalcorresponding to the actual heading of the vessel is provided. Theautopilot includes an operator actuable controller for providing asignal corresponding to a desired course for the vessel and means fed bythe actual heading signal and the desired course signal for generatingan error signal corresponding to the deviation between the desiredcourse and the actual heading. An intermediate rudder control signal isgenerated in response to the error signal. A comparator is provided forcomparing the intermediate rudder control signal to a deadband thresholdsignal. The autopilot further includes means for modifying theintermediate rudder control signal in response to the comparator,wherein the modified rudder control signal is related to the integral ofthe error signal when the intermediate rudder control signal is lessthan the deadband threshold signal.

With this arrangement, a marine autopilot is provided with a deadbandfeature which causes values of the intermediate rudder control signalless than the deadband threshold signal to be ignored. Moreparticularly, when the intermediate rudder control signal is within thedeadband limits, the modified rudder control signal is related to thetime integral of the error signal in order to compensate for lowfrequency disturbances such as wind.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following detaileddescription of the drawings in which:

FIG. 1 is a simplified block diagram of an adaptive autopilot inaccordance with the present invention;

FIG. 2 is a block diagram of the course change circuit of the autopilotof FIG. 1;

FIG. 3 is a block diagram of the course keeping circuit of the autopilotof FIG. 1;

FIG. 4 is a block diagram of the deadband control circuit in accordancewith the present invention;

FIG. 5 is a is a block diagram of a deadband determination circuit;

FIG. 6 simplified flow diagram of the adaptive deadband feature of thepresent invention; and

FIG. 7 is a sketch of a rudder order signal and the corresponding ruddercontrol signal in accordance with the adaptive deadband feature of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, an autopilot 10 is coupled to a heading sensorandprocessor unit 12, a speed sensor and processor unit 14, and a rollsensor and processor unit 16. The autopilot 10 is adapted for use on avessel (not shown), for example a marine vessel having a rudder 61. Theheading sensor and processor unit 12 is coupled to the vessel andprovides a heading signal, here carried by signal line 18, to autopilot10. Similarly, speed sensor and processor unit 14 is coupled to thevessel andprovides a velocity scheduling signal to autopilot 10 carriedby signal line 20 as will be described. Roll sensor and processor unit16, also coupled to the vessel, provides a roll signal to autopilot 10carried by signal line 22, as shown. More particularly, heading signalline 18 carries a signal representing the actual heading of the vessel.The velocity scheduling signal is related to the speed of the vesselover water and the roll signal represents the frequency and magnitude ofthe angle of the roll of the vessel.

Here, heading sensor and processor unit 12 includes a flux gate compass.More particularly, an INI-100 compass manufactured by InternationalNavigation, Inc. of Boca Raton, Fla. and having a five Hertz samplingrateis used. Note however that other compasses, or even a Loran or GPSdevice, may alternatively be used. Also, it may be desirable to use twosuch devices, such as a Loran and a compass, in order to provide headingsignalredundancy. The heading sensor and processor unit 12 may alsoinclude a filter in order to reduce the effect of noise on the autopilot10 operation introduced by the compass. Here, the heading signalsamples, andmore particularly five samples per second, are provided bythe compass to amicroprocessor (not shown) which rejects any signalsample that deviates from the previous sample by more than twentydegrees. If a signal sample is rejected, the previous heading signalsample replaces the rejected sample. Other filtering techniques, such asKalman filtering, may additionally or alternatively be used. Here, a68HC11 microprocessor sold by Motorola, Inc. of Shaumburg, Ill. is used,but any conventional microprocessor is suitable.

The speed sensor and processor unit 14 may comprise a paddle wheelsensor, a Loran, or GPS device. Here, a paddle wheel sensor is used toprovide a signal related to the vessel speed over water. The speedsensor and processor unit 14 may also include a filter in order toreduce the effect of noise on the autopilot 10 operation introduced bythe paddle wheel sensor, such as a "backward differences" filteringtechnique as described in a book entitled "Automatic Tuning of PIDControllers" authored by K. J.Astrom and T. Hagglund. Again, speedsignal redundancy may be provided by using more than one speedindicating device, such as a Loran or GPS devicein addition to a paddlewheel sensor.

The speed sensor and processor unit 14 also includes a velocityscheduling portion which provides the velocity scheduling signal of line20. More particularly, the velocity scheduling signal of line 20 isrelated to the speed of the vessel and, specifically, such velocityscheduling signal is inversely proportional to the vessel speed. Thevelocity scheduling portion of unit 14 includes a look-up tablecontaining a velocity scheduling signal value corresponding to values ofvessel speed. In other words, in response to the speed related signalprovided by the paddle wheel sensor and filtered as described above, avelocity scheduling value is provided on velocity scheduling signal line20. For example, if the vessel speed is seven knots, the velocityscheduling signal of line 20 mayhave a value of one; whereas, if thevessel speed is eleven knots, the signal of line 20 may have a value of0.5. The actual look-up table values(i.e. the values of the velocityscheduling signal corresponding to values of vessel speed) are afunction of the vessel's characteristics, such as hull shape and weight.

The roll sensor and processor unit 16 here, includes an "AccuStar II"dual axis clinometer manufactured by Lucas Sensing Systems of Phoenix,Ariz. The unit 16 here, also includes an analog to digital converter fordigitizing the analog output signal of the clinometer. Furthermore, asuitable filter may be provided in the roll sensor and processor unit 16in order to reduce the effect of noise on the autopilot 10 operationintroduced by the clinometer. Here, a third order Butterworth digitallow-pass filter is used to process the digitized clinometer outputsignal in order to provide the roll signal. The roll signal thusprovided is carried by signal line 22 to autopilot 10, as shown.

In operation of autopilot 10, the operator, or helmsmen of the vesselactuates a desired course input control 25 to indicate a desired coursefor the vessel. More particularly, signal line 26 carries a signalcorresponding to the desired vessel course. Heading signal line 18 anddesired course signal line 26 are coupled to a summing circuit 24, asshown, for providing an error signal e(t) corresponding to the deviationor difference between the desired course and the actual heading. Here,theerror signal e(t) is not permitted to exceed, for example thirtydegrees. The benefit of this error signal limiting feature is that sincethe error signal is used to compute the desired rudder position, orangle and hence the angular velocity with which the vessel moves as willbe described below, by limiting the error signal, the angular velocityof the vessel iseffectively limited.

Error signal line 28 is coupled to an input of a comparator circuit 30.A reference voltage V_(ref) is coupled to a second input of thecomparatorcircuit 30 by signal line 32, as shown. An output signal line34 of comparator circuit 30 carries a logic signal indicative of thelevel of the error signal e(t) relative to the reference voltageV_(ref). More particularly, if the error signal e(t) is greater in valuethan the reference voltage V_(ref), the logic signal of output signalline 34 is in a first logic state. However, if the error signal e(t) isless than thereference voltage V_(ref), the logic signal of line 34 isin a second logic state. Logic signal line 34 is coupled to a switch 36and controls the position thereof, as will be described below. Sufficeit here to say that when logic signal line 34 is in the first logicstate, switch 36 is in a first position 36a and when logic signal line34 is in the second logic state, switch 36 is in a second position 36b.

Error signal line 28, is further coupled to means 42 responsive to theerror signal for generating an intermediate rudder control signal online 48, or an intermediate rudder control signal generator 42. Moreparticularly, intermediate rudder control signal generator 42 includes acourse change circuit 38 and a course keeping circuit 40, as shown, towhich error signal line 28 is coupled. Also coupled to the course changecircuit 38 and the course keeping circuit 40 is the velocity schedulingsignal line 20. The operation of course change circuit 38 will bedescribed in conjunction with FIG. 2 and that of course keeping circuit40will be described in conjunction with FIG. 3. Suffice it here to saythat in response to the error signal e(t) and the vessel schedulingsignal, course change circuit 38 generates a course change signal at anoutput thereof, carried by signal line 44. Course keeping circuit 40 isfurther fed by the roll signal line 22, as shown. In response to theerror signal e(t), the velocity scheduling signal, and the roll signal,course keeping circuit 40 generates a course keeping signal at an outputthereof, carriedby signal line 46.

More particularly, the course change signal line 44 and the coursekeeping signal line 46 are coupled to switch 36, as shown. Switch 36 isfurther coupled to an intermediate rudder control signal line 48, asshown. When switch 36 is disposed in position 36a, the course changesignal line 44 iscoupled to the intermediate rudder control signal line48. When switch 36 is in position 36b, the course keeping signal line 46is coupled to the intermediate rudder control signal line 48. Thus, whenthe logic signal carried by line 34 is in the first logic state,intermediate rudder control signal line 48 is coupled to course changesignal line 44; whereas, when the logic signal of line 34 is in thesecond logic state, the course keeping signal line 46 is coupled tointermediate rudder control signal line 48. In this way, the magnitudeof the error signal e(t) governs which of signal lines 44 and 46provides the intermediate rudder control signal of line 48. Stateddifferently, when the error signal e(t) is greater than the referencevoltage V_(ref), the intermediate rudder control signal of line 48 isprovided by the course change signal of line 44. Whereas, when the errorsignal e(t) is less thanthe reference voltage V_(ref), the intermediaterudder control signal of line 48 is provided by the course keepingsignal of line 46. Here, the intermediate rudder control signal of line48 is updated ten times per second. Also, if the intermediate ruddercontrol signal is being provided by the course change signal and theerror signal e(t) has not improved (i.e. decreased) within fifteenseconds, then the switch 36 is disposed inposition 36b so that theintermediate rudder control signal is provided by the course keepingsignal.

Intermediate rudder control signal line 48 is coupled to a deadbandcontrolcircuit 50, as shown. Also coupled to deadband control circuit 50is the heading signal line 18, the error signal line 28, the velocityscheduling signal line 20, and the roll signal line 22. A signalgenerated by course keeping circuit 40, referred to hereinafter as theintegral term signal and carried by signal line 92, is also coupled tothe deadband control circuit 50, as shown. In response to theabove-mentioned signal lines coupled to deadband control circuit 50, arudder control signal is provided at an output of circuit 50 and iscarried by signal line 54. The manner of providing the rudder controlsignal of line 54 will be describedbelow in conjunction with FIGS. 4-7.Suffice it here to say that when the intermediate rudder control signalof line 48 is greater than a deadband threshold signal, or moreparticularly is outside of a deadband range defined by such deadbandthreshold signal, the rudder control signal of line 54 is set equal tothe intermediate rudder control signal of line 48.However, when theintermediate rudder control signal of line 48 is within such deadbandrange, the rudder control signal of line 54 is set equal to the integralterm signal of line 92, as will be described.

The rudder control signal provides the output signal of autopilot 10,via signal line 54, and is coupled to a motor controller 56, as shogun.In response to inter alia the rudder control signal, motor controller 56provides a motor drive signal at an output thereof carried by line 58.Themotor drive signal line 58 is further coupled to a motor 59, theoperation of which controls the position of the rudder 61.

More particularly, the rudder control signal line 54 is coupled to aproportional plus derivative controller 60 of motor controller 56. Alsocoupled to proportional-derivative controller 60 is a rudder anglesignal line 62, as shown. Specifically, a rudder sensor unit 64 iscoupled to therudder 61 and generates the rudder angle signal of line 62in response to the position of the rudder 61. Here, rudder sensor unit64 is a potentiometer. Proportional plus derivative controller 60provides a digital output signal, via signal line 66, representative ofthe speed anddirection with which the rudder 61 should move to maintainor change the vessel to the desired course. More particularly,consecutive samples of the rudder control signal of line 54 are storedin a memory device. Here, such consecutive samples are separated in timeby approximately 0.1 seconds. The controller 60 computes the differencebetween two consecutivesamples and the difference thus provided isdivided by 0.1 seconds in orderto provide a signal hereinafter referredto as the motor controller derivative term signal. Also within thecontroller 60, the rudder angle signal is subtracted from the ruddercontrol signal, the latter being interpolated from consecutive samplesof the rudder control signal of line54 at a rate of 250 cycles/second.The difference thus provided may be multiplied by a gain value toprovide a signal, hereinafter referred to asthe motor controllerproportional term signal. The motor controller derivative andproportional term signals are then added to provide the output signal ofproportional plus derivative controller 60 via signal line 66.

Signal line 66 is coupled to a digital-to-analog converter 68 whichprovides an analog replica of the digital signal of line 66 at an outputthereof. The analog signal output of line 58 is referred to as the motordrive signal and is fed to the rudder motor 59 as mentioned above.

Referring now to FIG. 2, the course change circuit 38 provides thecourse change signal of line 44 at an output thereof, as shown. Asmentioned above, when the error signal e(t) is greater than thereference voltage V_(ref), such course change signal of line 44 providesthe intermediate rudder control signal of line 48 (FIG. 1). In otherwords, course change circuit 38 governs the position of the vessel'srudder 61 when the error signal e(t) is greater than the referencevoltage V_(ref).

Here, course change circuit 38 is a proportional plus derivativecontroller. More particularly, error signal line 28 is coupled to adifferentiator 70 and to a multiplier 72. Also coupled to multiplier 72and to a second multiplier 74 is the velocity scheduling signal line 20.Differentiator 70 provides an output signal, carried by signal line 76,corresponding to the time rate of change of the error signal e(t) (i.e.e(t)). Such time rate of change, or derivative, of the error signal maybefiltered by any known technique such as a "backward differences"technique mentioned above and is coupled to multiplier 74, as shown.

Multiplier 72 has a signal line 78 carrying a proportional gain valueK_(p) coupled thereto. Multiplier 72 multiplies the error signal e(t)bythe proportional gain value K_(p) and the velocity scheduling signalof line 20. The product thus computed provides a signal, carried bysignal line 80, and referred hereinafter as the proportional termsignal. The proportional gain constant K_(p) is a function of thevessel's characteristics, such as weight and hull shape and, here, isdetermined atcruising speed when the autopilot 10 is installed. Forexample, when autopilot 10 is installed on a thirty-five foot,semi-displacement, singlepropeller vessel, the cruising speed isapproximately seven knots and the proportional gain value isapproximately 0.25.

Multiplier 74 multiplies the derivative e(t) of the error signal carriedbyline 76 with the velocity scheduling signal of line 20 and furtherwith a counter rudder gain value K_(d) carried by signal line 82, asshown. Here, like the proportional gain value K_(p), the counter ruddergain value K_(d) is determined at cruising speed when the autopilot 10is installed. For example, on the above-described boat, the counterrudder gain value is K_(d) is approximately 0.25. The product thusprovided, orthe output of multiplier 74, is carried by signal line 84and will hereinafter be referred to as the derivative term signal.

Signal lines 80 and 84, carrying the proportional term signal and thederivative term signal respectively, are coupled to inputs of a summingcircuit 86 as shown, the output of which provides the course changesignalof line 44 (FIG. 1). In other words, the course change signal isprovided by the summation of the proportional term signal of line 80 andthe derivative term signal of line 84.

Referring now to FIG. 3, the course keeping circuit 40 is shown. Coupledtocourse keeping circuit 40 is the error signal line 28, the velocityscheduling signal line 20, and the roll signal line 22. Course keepingcircuit 40 provides, at an output thereof, the course keeping signal ofline 46. More particularly, the course keeping signal of line 46 isgenerated at an output of a summing circuit 116 in response to inputsignal lines 122 and 98 and signal line 122 is generated at an output ofasumming circuit 90 in response to input signal lines 92, 94, and 96.Signalline 92 carries a signal proportional to the time integral of theerror signal, and will hereinafter be referred to as the integral termsignal line 92. Signal line 94 carries a signal proportional to theerror signal and will thus be referred to as the proportional termsignal line 94. Signal line 96 carries a signal proportional to the timerate of change, or derivative, of the error signal and will be referredto as the derivative term signal line 96. Signal line 98 carries asignal proportional to the roll angle of the vessel and will be referredto as the roll term signal line 98. The way in which each of theabove-referenced signals is generated will now be described.

The proportional term signal carried by line 94 is equivalent to theproportional term signal carried by signal line 80 of course changecircuit 38 (FIG. 2). More particularly, a multiplier 100 is fed by errorsignal line 28 and velocity scheduling signal line 20. Multiplier 100 isfurther fed by a signal line 102 carrying a signal representing theproportional gain value K_(p). Multiplier 100 multiplies the errorsignal e(t) with the proportional gain value K_(p) and the velocityscheduling signal of line 20 to provide the proportional term signal ofline 94.

Error signal line 28 is also coupled to a differentiator 104, as shown.Theoutput of differentiator 104 is provided by signal line 106 andcorrespondsto the time rate of change, or the derivative e(t), of theerror signal. Signal line 106 is coupled to a multiplier circuit 108, asshown. Also coupled to multiplier 108 is the velocity scheduling signalline 20 and a counter rudder gain value K_(d) via signal line 110. Thecounter rudder gain value K_(d) is, here, equivalent to that of signalline 82 (FIG. 2). In addition to the above-mentioned inputs tomultiplier 108, such inputs being equivalent to those of multiplier 74(FIG. 2), multiplier 108has a fourth input provided by a roll frequencyscheduling circuit 112. More particularly, the roll frequency schedulingcircuit 112 provides a roll frequency scheduling signal to multiplier108 via signal line 114, asshown, for adjusting the course keepingsignal of line 46 as will be described. Multiplier 108 multiplies thederivative e(t) of the error signal of line 106, the velocity schedulingsignal of line 20, the counterrudder gain value K_(d) of line 110, andthe roll frequency scheduling signal of line 114 to generate an outputsignal via line 96. In other words, such multiplier 108 provides asignal proportional to the derivative of the error signal (i.e. thederivative term signal).

As mentioned, roll frequency scheduling circuit 112 provides a rollfrequency scheduling signal via signal line 114 which is used bymultiplier 108 to generate the derivative term signal of line 96. Moreparticularly, the roll frequency scheduling circuit 112 is fed by therollsignal line 22, as shown. In response to the frequency of thevessel's roll, the roll frequency scheduling circuit 112 generates aroll frequencyscheduling signal having a value between zero and one onsignal line 114. The roll frequency scheduling signal thus modifies thederivative term signal of line 96 as a function of the roll frequency ofthe vessel. When the vessel is in following seas, as determined by theroll frequency beingbelow a first predetermined value, such as 0.25Hertz, the roll frequency scheduling signal has a value of one; whereas,when the vessel is heading into the seas, as determined by the rollfrequency being above a second predetermined value, such as 0.45 Hertz,the roll frequency scheduling signal has a value of zero. Furthermore,for values of roll frequency between the first and second predeterminedvalues, the corresponding valueof the roll frequency scheduling signalis, here provided by linear interpolation. With this arrangement, thederivative term signal of line 96 is nulled, or set to zero, when thevessel is heading into the seas andis equal to the product of thederivative e(t) of the error signal, the velocity scheduling signal ofline 20, and the counter rudder gain value K_(d) in following seas. Asmentioned above, the derivative term signal of line 96 provides systemdamping in the sense that it provides resistance to the vessel's angularvelocity. While the damping effect of the derivative term is desirablein following sea conditions in order to prevent overshoot of the vesselrelative to the desired course, when the vessel is heading into theseas, the damping thus provided is ineffective.In other words, sincedisturbances in the vessel's heading tend to occur ata relatively highfrequency when the vessel is heading into the waves, or seas, thedamping provided by the derivative term signal does relatively little tokeep the vessel on the desired course. Thus, since the effect ofthederivative term is negligible in such conditions, and moreover, due tothe wear on the rudder 61 and associated drive apparatus caused byineffectual movement thereof, the derivative term signal is eliminated,ornulled when the vessel is heading into the seas.

Note that the roll frequency scheduling circuit 112 is not used duringcourse change operation (FIG. 2). This is simply due to the fact thatduring course changes, the boat will roll. Thus, nulling the derivativeterm signal when the roll is above the predetermined value (i.e. as canbeachieved with circuit 112) would tend to null the derivative termcontinuously during course change operation.

The integral term signal of line 92 is provided at an output of asumming circuit 136. More particularly, a first summing circuit 124 hasa first input provided by signal line 122. As mentioned above, signalline 122 is provided at the output of summing circuit 90 and isequivalent to the summation of the proportional term signal of line 94,the derivative term signal of line 96, and the integral term signal ofline 92. A second inputto summing circuit 124 is provided by theintegral term signal of line 92, as shown. More particularly however,the signal provided by line 92 to an input of summer 124 represents asample of the integral term signal taken at, or corresponding to, timet-1. Note that the same signal (i.e. representing a sample at time t-1)is provided at an input to summing circuit 136, by signal line 92.Whereas, signal line 92 as coupled to an input of summing circuit 90 andas coupled to deadband control circuit 50 (FIG. 1) represents a sampleof the integral term signal taken at, or corresponding 40, time t. Inother words, designating the integral term signal as I(t), I(t-1) iscoupled to an input of summing circuits 124 and 136, whereas I(t) iscoupled to summing circuit 90 and to deadband controlcircuit 50. Thus,the output signal line 128 of summing circuit 124 can be expressed asbeing equal to I(t-1)-P(t)-D(t)-I(t) where P(t) is the proportional termsignal of line 94 and D(t) is the derivative term signalof line 96.

Signal line 128 is coupled to a multiplier circuit 130. Also coupled tomultiplier circuit 130 is a trim value carried by signal line 132, asshown. The trim value is a function of the vessel's characteristics suchas hull shape and weight. For example, when autopilot 10 is used on athirty-five foot, semi-displacement, single propeller vessel, the trimvalue is approximately 0.04 degrees of rudder per second. The output ofmultiplier 130 is carried by signal line 134 and represents the productofthe signal of line 128 with the trim value. Signal line 134 is coupledto summing circuit 136, such circuit 136 also having the integral termsignalline 92 coupled thereto, as mentioned. Thus, the integral termsignal provided to summer 90 can be expressed as I(t)=I(t-1)+Trim Value[I(t-1)-P(t)-D(t)-I(t)].

The roll term signal carried by line 98 to summing circuit 116 isprovided as follows. Roll signal line 22, here provided by the rollsensor and processor unit 16 (FIG. 1), is coupled to an averagingcircuit 146 and to a summing circuit 148, as shown. Note that the rollsignal of line 22 represents the instantaneous value of the angle ofroll of the vessel. Averaging circuit 146 is used to average samples ofthe instantaneous rollsignal provided thereto, here, over a two minuteperiod. An output signal line 142 of averaging circuit 146, carrying theaveraged roll signal, is also coupled to summing circuit 148. Summingcircuit 148 subtracts the instantaneous roll signal of line 22 from theaveraged roll signal of line142 to provide a normalized roll signalcarried by a signal line 150 to a multiplier 140. Also coupled tomultiplier 140 is a roll gain value via a signal line 152, as shown. Thevalue of roll gain may be manually entered and adjusted by the operator,or helmsmen of the autopilot 10. Alternatively, the value of roll gainmay be adaptively, or automatically computed by autopilot 10. Moreparticularly, such an adaptive roll gain value would be initially set atinstallation of the autopilot 10 as a function of the vessel'scharacteristic such as weight and hull shape and then, during operation,would be modified automatically from the installation value bymultiplying such installation value by the velocity scheduling signal ofline 20. In this way, the roll term signal will be increased as thespeed of the boat is decreased (since the velocity scheduling signal isinversely proportional to the speed of the vessel). It may be furtherdesirable to multiply the product of the roll gain valueand the velocityscheduling signal by the roll frequency scheduling signal of line 114.In other words, since compensating for the roll of the vessel(i.e. asachieved by the roll term signal of line 98) is ineffective when headinginto the seas, it may desirable to null the roll term signal bymultiplying the roll gain value by the roll frequency scheduling signalofline 114. Since the value of the counter rudder signal isapproximately onein beam sea conditions, the value of the roll termsignal will be unaffected in such conditions.

Note that it is desirable to provide the course keeping signal as afunction of the roll angle of the vessel since rolling of the vesselprecedes deviation of the vessel from the desired course. In other wordsby compensating for the roll angle of the vessel, tighter course keepingis provided. It is desirable to use the magnitude of the roll angle inthis capacity due to the simplicity of sensing the roll angle (i.e. withthe clinometer as mentioned above). Stated differently, use of the rollangle to provide the course keeping signal effectively reduces theresponse time of the autopilot 10. Moreover, the magnitude of the rollangle is desirable since other representations of the vessel's roll, forexample the derivative of the roll angle, tend to be noisy and thusintroduce noise into the system.

Referring to FIGS. 1-3, it should be noted that the course changecircuit 38 and the course keeping circuit 40 may be combined in theimplementationof autopilot 10. In other words, here a separateimplementation of circuits38 and 40 is shown in FIGS. 1-3 in order tofacilitate understanding of their operation. In particular, it isconvenient to view the relationship between the error signal and thereference voltage (FIG. 1) in terms of selecting one or the other ofcircuits 38 and 40 to control the vessel's rudder. However, a combinedimplementation of circuits 38 and 40 is readily achieved since theproportional term signals associated with the course change circuit 38and the course keeping circuit 40 (i.e. signal line 94 of FIG. 3 andsignal line 80 of FIG. 2) are identical. Moreover, the derivative termsignal of line 96 (FIG. 3) differs from the derivativeterm signal ofline 84 (FIG. 2) only in the addition of the roll frequency schedulingsignal of line 114 (FIG. 3). Additionally, during coursechangeoperation, the integral term signal of line 92 (FIG. 3) and theroll term signal of line 98 (FIG. 3) could be nulled if circuits 38 and40 were combined in the implementation of autopilot 10.

Referring now to FIG. 4, a block diagram of deadband control circuit 50(FIG. 1) is shown to include a response deadband circuit 156 and a noisedeadband circuit 158. The rudder control signal of line 54 (FIG. 1) isprovided at an output of the noise deadband circuit 158, as shown.Circuits 156 and 158 represent, generally, subsequent deadbandprocedures or operations by which the rudder control signal of line 54is generated, as will now be described. Note that here, circuits 156 and158 operate to update the rudder control signal of line 54 ten times persecond.

The intermediate rudder control signal line 48 and the integral termsignalline 92 are coupled to the response deadband circuit 156, asshown. Also coupled to the response deadband circuit 156 is a responsedeadband threshold signal carried by signal line 162. The responsedeadband threshold signal line 162 is generated by a response deadbandthreshold circuit 164, as shown and is updated once every thirtyseconds. An output signal line 160 of response deadband circuit 156 iscoupled to noise deadband circuit 158 and carries a signal hereinafterreferred to as the modified rudder control signal.

More particularly, the response deadband threshold signal of line 162 isgenerated by circuit 164 in response the velocity scheduling signal line20, the error signal line 28, and the roll signal line 22, such signallines being coupled to inputs thereof. The way in which responsedeadband threshold circuit 164 generates the response deadband thresholdsignal will be described below in conjunction with FIGS. 5 and 6.Suffice it hereto say that the response deadband threshold signal ofline 162 represents aresponse deadband value, corresponding to aresponse deadband range used bythe response deadband circuit 156 togenerate the modified rudder control signal of line 160, as will now bedescribed.

Referring now to the response deadband circuit 156, the responsedeadband threshold signal line 162 and the intermediate rudder controlsignal line 48 are coupled to inputs of a comparator circuit 166. Theoutput signal line 170 of comparator circuit 166 is a logic signalprovided in a first logic state when the intermediate rudder controlsignal is greater in value than the response deadband threshold signal.The logic signal of line 170 is provided in a second logic state whenthe intermediate rudder control signal is less than the responsedeadband threshold signal. More specifically, comparator circuit 166more accurately represents a window comparator arrangement such thatoutput signal 170 is in the first logic state when the intermediaterudder control signal is outside of the response deadband range and isin the second logic state if the intermediate rudder control signal iswithin the response deadband range. The response deadband range is arange centered about the value of the integral term signal and equal inmagnitude to twice the value of the response deadband signal. In otherwords such response deadband range is defined by an upper responsedeadband limit and a lower response deadband limit, such limitsseparated by twice the value of the response deadband threshold signalof line 162. Logic signal line 170 is coupled to a switch168 to providesuch switch 168 in a first position 168a, as shown, when thelogic signalof line 170 is in the first state and in a second position 168b when thelogic signal of line 170 is in the second state.

Also coupled to switch 168 is the integral term signal line 92, theintermediate rudder control signal line 48, and the modified ruddercontrol signal line 160, as shown. With this arrangement, when theintermediate rudder control signal exceeds, or is outside of, theresponsedeadband range, the modified rudder control signal line 160 iscoupled to the intermediate rudder control signal of line 48. Whereas,when the intermediate rudder control signal is within the responsedeadband range, the modified rudder control signal of line 160 iscoupled to the integral term signal of line 92. In other words, themodified rudder control signalof line 160 is equal to the intermediaterudder control signal of line 48 when the intermediate rudder controlsignal exceeds the response deadband limits (i.e. is greater inmagnitude than the response deadband threshold signal). However, themodified rudder control signal of line 160 is equal to the integral termsignal of line 92 when the intermediate rudder control signal is withinthe response deadband limits (i.e. is smaller in magnitude than theresponse deadband threshold signal). Note that once theintermediaterudder control signal exceeds the response deadband limits, the modifiedrudder control signal is provided by the intermediate rudder controlsignal until such intermediate rudder control signal crosses theintegral term signal and is within the response deadband limits. Thisarrangement ensures that the vessel will be brought back to the desiredcourse when it is heading into the seas and the response deadband rangeisrelatively large. These conditions will become more clear from thedescription of FIG. 7 below.

It is desirable to provide the modified rudder control signal of line160 in accordance with the response deadband threshold signal of line162 as described above since, in certain conditions, rudder movement isineffectual in maintaining or keeping the vessel on the desired course.More particularly, when the vessel is heading into the direction of thewaves, the perturbations in the vessel's heading will occur at arelatively high frequency. In fact, the frequencies of suchperturbations tend to be so high that the rudder 61 cannot move fastenough to compensate for them. Thus, a signal directing the movement ofthe rudder'sposition in response to such perturbations will beineffectual in maintaining the vessel on the desired course. Moreover,the movement of the rudder 61 in response to such a signal will resultin unnecessary wearon the rudder 61 and associated drive apparatus.Response deadband circuit 156 sets the modified rudder control signal ofline 160 equal to the intermediate rudder control signal of line 48 whenthe conditions are suchthat the rudder 61 can effectively respond to theintermediate rudder control signal of line 48 in order to maintain thedesired course. However, when conditions are such that the rudder 61cannot effectively respond to the intermediate rudder control signal ofline 48 in order to maintain the desired course, circuit 156 sets themodified rudder control signal of line 160 equal to the integral termsignal of line 92.

Having the modified rudder control signal of line 160 equal to theintegralterm signal of line 92 when the rudder 61 cannot compensate forperturbations in the vessel's heading provides improved short-termcourse keeping capability. More particularly, the integral term signalcompensates for low frequency disturbances in the vessel's heading, suchas wind. Thus, by having the modified rudder control signal equal theintegral term signal, such low frequency disturbances will becompensated in conditions where the rudder 61 cannot effectivelycompensate for heading disturbances. In other words, if the modifiedrudder control signal was held constant when the value of such signal iswithin the response deadband range, a small heading deviation may existfor a relatively long period of time before the disturbance becomeslarge enoughto force the intermediate rudder control signal outside ofthe response deadband range. In contrast, in the present system in whichthe modified rudder control signal is equal to the integral term signalwhen the intermediate rudder control signal is within the responsedeadband range, such low frequency disturbances are continuouslycompensated. This arrangement ensures that the vessel will remain oncourse when subjected to low frequency disturbances, even when theresponse deadband is relatively large.

As mentioned, modified rudder control signal 160 is coupled to the noisedeadband circuit 158. The noise deadband circuit 158 operates on themodified rudder control signal to provide the rudder control signal ofline 54. More particularly, signal line 160 is coupled to an input of acomparator circuit 172. Also coupled to an input of comparator circuit172is a noise deadband threshold signal carried by signal line 174. Thenoise deadband threshold signal corresponds to a noise deadband value,or limit which is set during the installation of autopilot 10. In otherwords, here, the noise deadband threshold signal has a constant value.Here, the noise deadband value is approximately 0.25. However, the noisedeadband range thus provided (i.e. a range equal to twice the value ofthe noise deadband threshold signal) is scaled up or down as a functionof the rudder control signal of line 54, as will be described. In otherwords, the noise deadband range is defined by an upper noise deadbandlimit and alower noise deadband limit.

The output of comparator circuit 172, here representing a windowcomparatorarrangement, is a logic signal carried by signal line 176 andcoupled to a switch 178 for controlling the position thereof. Moreparticularly, the logic signal of line 176 is provided in a first logicstate when the modified rudder control signal exceeds, or is outside of,the noise deadband range. Such logic signal is in a second logic statewhen the modified rudder control signal is within the noise deadbandrange. In response to signal line 176, switch 178 is disposed in a firstposition 178a or in a second position 178b to provide the rudder controlsignal of line 54.

More particularly, the rudder control signal line 54 is coupled to anoutput of switch 178 and is also fed back to an input of switch 178,such input being further coupled to a noise deadband threshold circuit179 as shown. Also coupled to an input of switch 178 is the modifiedrudder control signal line 160. The noise deadband threshold circuit 179scales the noise deadband range up or down in accordance with the ruddercontrol signal. More particularly, circuit 179 scales the noise deadbandrange up or down in order to center such range around the rudder controlsignal of line 54, as will become more clear in conjunction with thedescription of FIG. 7 below. With noise deadband circuit 158 asdescribed, when switch 178 is in the first position 178a, the ruddercontrol signal of line 54 isequal to the modified rudder control signalof line 160 minus the value of the noise deadband threshold signal. Whenswitch 178 is in the second position 178b, the rudder control signal ismaintained constant.

One purpose of noise deadband circuit 158 is to minimize rudder activitycaused by noise, such noise being introduced by the heading sensor andprocessor unit 12 (FIG. 1), the speed sensor and processor unit 14 (FIG.1), and the roll sensor and processor unit 16 (FIG. 1). Another purposeofthe noise deadband circuit 158 is to compensate for inaccuries due tomechanical restraints and caused by the motor controller 56 (FIG. 1).Generally, the noise deadband range is significantly smaller than theresponse deadband range as will become apparent from the description ofFIG. 7 below. It should be appreciated however that due to the coursekeeping accuracy realized with the use of the response deadbandoperation,and further in view of advances in heading detection, incertain applications it may be suitable to eliminate the noise deadbandoperation.

Referring now to FIG. 5, the response deadband threshold circuit 164provides the response deadband threshold signal of line 162 at an outputthereof, as shown, in response to the velocity scheduling signal of line20, the error signal line 28, and the roll signal line 22. Moreparticularly, once every thirty seconds, four different deadband valuesare computed by response deadband threshold circuit 164. These deadbandvalue signals are carried by a MAXIMUM DEVIATION deadband signal line180,a MAXIMUM ANGULAR VELOCITY deadband signal line 182, a FIXED LIMITdeadbandsignal line 184, and a VARYING LIMIT deadband signal line 186.Before discussing the way in which these four deadband signals are usedto provide the response deadband threshold signal of line 162, thecomputation of each of the deadband signals will now be described.

The MAXIMUM DEVIATION deadband signal of line 180 is provided at anoutput of a multiplier 188, as shown. More particularly, the MAXIMUMDEVIATION deadband signal is equivalent to the product of theproportional gain value K_(p) carried by signal line 190, the errorsignal (or average yawamplitude) carried by signal line 28, the velocityscheduling signal carried by signal line 20, and a signal related to theroll frequency of the vessel and carried by signal line 192. Theproportional gain value K_(p) is as described above in conjunction withthe course change circuit 38 of FIG. 2 and the course keeping circuit 40of FIG. 3. The error signal input to multiplier 188 provides arepresentation of the vessel's yaw amplitude. Here, the error signal isaveraged to provide an average value of yaw amplitude. The velocityscheduling signal of line 20 is as described above in conjunction withFIGS. 1-4. The signal carried byline 192 may be referred to as a rollfrequency scheduling signal and is related to the roll signal of line 22as will now be described.

Roll signal line 22 is coupled to a roll frequency scheduling circuit194, like circuit 112 (FIG. 3). More particularly, signal line 192 isprovided at an output of the roll frequency scheduling circuit 194 andis related to the roll signal of line 22 by a look-up table arrangement.When the vessel is in following seas, the roll frequency of the vesselis relatively low. In such conditions, the roll frequency schedulingsignal of line 192 is set to zero. More particularly, a value of rollfrequency less than approximately 0.25 Hertz corresponds to a zero valueof the rollfrequency scheduling signal of line 192. In this way, theMAXIMUM DEVIATIONdeadband signal of line 180 is nulled, or set equal tozero, in such following sea conditions. When the vessel is heading intothe seas, the roll frequency thereof is relatively high. In theseconditions, and more particularly, when the vessel's roll frequency toabove approximately 0.45Hertz, the roll frequency scheduling signal ofline 192 is set to a value of approximately one. In this way, when thevessel is heading into the waves, the MAXIMUM DEVIATION deadband signalof line 180 is equal to the product of the proportional gain valueK_(p), the error signal, and the velocity scheduling signal. Moreover,here linear interpolation is used toprovide the value of the rollfrequency scheduling signal for roll frequencies between 0.25 Hertz and0.45 Hertz.

The MAXIMUM ANGULAR VELOCITY deadband signal of line 182 is provided atan output of a multiplier circuit 196. More particularly, the MAXIMUMANGULARVELOCITY deadband signal is equal to the product of a rollfrequency scheduling signal carried by line 202, an angular velocitysignal of line 198, the velocity scheduling signal of line 20, and thecounter rudder gain value K_(d) carried by signal line 200. Here, theroll frequency scheduling signal of line 202 is equal to the square ofthe value of the roll frequency scheduling signal of line 192. Theangular velocity signal of line 198 is provided at an output of adifferentiator 204, as shown. Differentiator 204 is fed by the errorsignal line 28 and the roll signal line 22. As above, the error signalline 28 provides a signal corresponding to the vessel's yaw amplitude. Asignal having an amplitude equivalent to the vessel's yaw amplitude anda frequency equivalent to thevessel's roll frequency is differentiatedby differentiator 204 to provide the angular velocity signal of line198. The counter rudder value K_(d) provided to multiplier 196 by signalline 200 is the same as that described above in conjunction with FIGS. 2and 3.

With this arrangement, when the roll frequency scheduling signal of line202 has a value of zero (i.e. when the vessel is heading into the seas),the MAXIMUM ANGULAR VELOCITY deadband signal of line 182 is zero. On theother hand, when the roll frequency scheduling signal of line 202 has avalue of one (i.e. when the vessel is in following seas), the MAXIMUMANGULAR VELOCITY deadband signal of line 182 is equal to the product ofthe angular velocity signal of line 198, the velocity scheduling signalofline 20, and the counter rudder gain value K_(d) carried by signalline 200.

MAXIMUM DEVIATION deadband signal line 180 and MAXIMUM ANGULAR VELOCITYdeadband signal line 182 are coupled to inputs of a comparator circuit206, as shown. In response to the signals carried by lines 180 and 182,comparator circuit 206 selects the larger (i.e. greater magnitude) ofthe MAXIMUM DEVIATION deadband signal and the MAXIMUM ANGULAR VELOCITYdeadband signal to provide an output signal carried by signal line 208.Output signal line 208 is coupled to an input of a second comparatorcircuit 210 as shown, the output of which provides the response deadbandthreshold signal of line 162. Also, coupled to inputs of comparatorcircuit 210 are the FIXED LIMIT deadband signal line 184 and the VARYINGLIMIT deadband signal line 186.

The FIXED LIMIT deadband signal is provided at an output of a multipliercircuit 212, as shown. More particularly, multiplier 212 is fed by thevelocity scheduling signal line 20 and multiplies such signal by aconstant value of 2.5 degrees rudder to provide the FIXED LIMIT deadbandsignal. Thus, the FIXED LIMIT deadband signal is proportional to thevelocity scheduling signal, which in turn is inversely proportional tothespeed of the vessel, as described above in conjunction with FIG. 1.

The VARYING LIMIT deadband signal of line 186 is provided at an outputof amultiplier circuit 214, as shown. The inputs of multiplier circuit214 are provided by the velocity scheduling signal line 20 and the rollsignal line 22. The VARYING LIMIT deadband signal is computed bymultiplier 214 as the product of the velocity scheduling signal, theroll signal, and a constant value of 2.5 degrees rudder. Here, the valueof the VARYING LIMITdeadband signal is varied in accordance with thetime during which the intermediate rudder control signal of line 48exceeds the response deadband range. More particularly, the VARYINGLIMIT deadband signal is decreased if the time during which theintermediate rudder control signal of line 48 is outside of the responsedeadband range exceeds the reciprocal of the roll frequency of thevessel. With this arrangement, theVARYING LIMIT deadband signal isdecreased if the vessel does not travel through the desired courseduring each roll cycle. Further, such value is increased if this time,during which the intermediate rudder control signal of line 48 isoutside of the response deadband range, is less than the reciprocal of,here, approximately four times the roll frequency. In other words, thiscondition of the value of the intermediate rudder control signal beingoutside of the response deadband range for a time less than thereciprocal of four times the roll frequency, indicates over use of therudder 61. Stated differently, such condition indicates that the rudder61 is being moved to attempt to compensate for relatively high frequencydisturbances which, in fact, the rudder 61 cannot effectivelycompensate. Thus, with this arrangement, VARYING LIMIT deadband signalwill be increased and, in the case where such VARYING LIMIT deadbandsignal provides the response deadband signal, the response deadbandlimitsare concomitantly increased.

As mentioned above, signal lines 184, 186, and 208 are coupled to inputsofcomparator circuit 210 to provide the response deadband thresholdsignal ofline 162. More particularly, comparator circuit 210 comparesthe relative levels of the signals carried by lines 184, 186, and 208and selects the signal of line 208 to provide the response deadbandthreshold signal unless such signal has a greater value than either ofthe limit deadband signals of line 184 or 186. In the case where thesignal of line 208 is greater than either the FIXED LIMIT deadbandsignal of line 184 or the VARYING LIMIT deadband signal of line 186,comparator circuit 210 selects the smaller of these limit deadbandsignals (i.e. from signal line 184 or 186) to provide the responsedeadband threshold signal of line 162.

With this arrangement, the response deadband threshold signal is afunctionof either the permissible MAXIMUM DEVIATION of the vessel fromits desired course (i.e. when the MAXIMUM DEVIATION deadband signal isgreater in value than the MAXIMUM ANGULAR VELOCITY signal and less thanboth the FIXED LIMIT signal and the VARYING LIMIT signal) or thepermissible MAXIMUM ANGULAR VELOCITY of the vessel (i.e. when theMAXIMUM ANGULAR VELOCITY deadband signal is greater in value than theMAXIMUM DEVIATION deadband signal and less than both the FIXED LIMITsignal and the VARYING LIMIT signal). However, if the greater of thedeadband signals of lines 180 and 182 is greater than either of thelimit deadband signals of lines 184 and 186, the response deadbandthreshold signal is provided by the limit deadband signal having thesmaller value.

Referring now to FIG. 6, the operation of response deadband thresholdcircuit 164 will be further described in conjunction with process stepsasmay be realized with a computer and a suitable software program. Step220 represents the beginning of a deadband computation. Moreparticularly, step 220 is executed once every thirty seconds. Insubsequent process step222, the MAXIMUM DEVIATION deadband signal ofline 180 is computed, as described above in conjunction with FIG. 5.More particularly, the MAXIMUMDEVIATION deadband signal is provided bymultiplying the proportional gain value K_(p), the error signal, thevelocity scheduling signal, and the roll related signal. The MAXIMUMANGULAR VELOCITY deadband signal is computed in step 224, again asdescribed above. In step 226, the FIXED LIMIT deadband signal iscomputed by multiplying the velocity scheduling signal of line 20 by aconstant value of 2.5 degrees rudder, as described above. Similarly, instep 228 the VARYING LIMIT deadband signal is computed, as describedabove.

Subsequently, in step 230, it is determined whether the MAXIMUMDEVIATION signal is greater than or equal to the MAXIMUM ANGULARVELOCITY signal andless than or equal to both the FIXED and VARYINGLIMIT deadband signals. Inother words, such determination is asdescribed above in conjunction with comparator circuits 206 and 210 ofFIG. 5. In the case where such determination is affirmative, theresponse deadband threshold signal is set equal to the MAXIMUM DEVIATIONsignal in step 232. However, if the MAXIMUM DEVIATION signal is notgreater than or equal to the MAXIMUM ANGULAR VELOCITY signal and lessthan the FIXED and VARYING LIMIT signals,step 234 is next executed inwhich it is determined whether the MAXIMUM ANGULAR VELOCITY signal isgreater than the MAXIMUM DEVIATION signal and less than or equal to boththe FIXED and VARYING LIMIT signals. If the determination in step 234 isaffirmative, step 236 is next executed in which the response deadbandthreshold signal is set equal to the MAXIMUM ANGULAR VELOCITY signal.Whereas, in the case where the MAXIMUM ANGULAR VELOCITY signal is notgreater than the MAXIMUM DEVIATION signal and is not less than or equalto both the FIXED and VARYING LIMIT signals, step 238 is next executed,as shown. In step 238, it is determined whether the FIXED LIMIT signalis less than the MAXIMUM DEVIATION signal or the MAXIMUM ANGULARVELOCITY signal and is less than the VARYING LIMIT signal.If suchdetermination is affirmative, step 240 is next executed in which theresponse deadband threshold signal is set equal to the FIXED LIMITsignal. However, if the determination of step 238 is negative, step 242isnext executed in which the response deadband threshold signal is setequal to the VARYING LIMIT signal.

Subsequent to steps 232, 236, 240, and 242, step 244 is executed inwhich it is determined whether the intermediate rudder control signal ofsignal line 48 is within the response deadband range. More particularly,step 244represents the functionality of the response deadband circuit156 (FIG. 4).Consider first the case where the intermediate ruddercontrol signal is notwithin, but exceeds the response deadband range. Inthis case, step 246 is next executed in which the modified ruddercontrol signal of line 160 (FIG. 4) is provided by, or set equal to theintermediate rudder control signal of line 48. If, however, theintermediate rudder control signal is determined to be inside of theresponse deadband range in step 244, step 248 is next executed in whichthe modified rudder control signal is set equal to the integral termsignal of line 92 (FIG. 4).

Subsequent to steps 246 and 248, step 250 is executed, such steprepresenting the functionality of the noise deadband circuit 158 (FIG.4).If it is determined, in step 250, that the modified rudder controlsignal is within the noise deadband range, step 252 is next executed inwhich therudder control signal of line 54 (FIG. 4) is held, ormaintained, constant.If, however, it is determined in step 250 that themodified rudder control signal is not within the noise deadband range,step 254 is next executed in which the rudder control signal of line 54is provided by, or set equalto the modified rudder control signal ofline 160 minus the value of the noise deadband threshold signal, as willbecome more clear from the description of FIG. 7 below. Subsequent tostep 254, step 256 is executed in which the noise deadband range isscaled up or down in accordance with the rudder control signal by thenoise deadband threshold circuit 179 (FIG. 4). Subsequent to steps 252and 256, a deadband computation is cyclecompleted.

Referring now to FIG. 7, and also to FIG. 4, the operation of deadbandcontrol circuit 50 will become more clear. FIG. 7 shows the relationshipbetween the response deadband range (i.e. corresponding to the responsedeadband threshold signal of line 162 and defined by the upper responsedeadband limit and the lower response deadband limit) and the noisedeadband range (corresponding to the noise deadband threshold signalcarried by signal line 174 and defined by the upper noise deadband limitand the lower noise deadband limit). Also shown on FIG. 7 is theintermediate rudder control signal of line 48, the modified ruddercontrolsignal of line 160, and the rudder control signal of line 54.

Referring first to the time period before time t₁, it can be seenthatthe intermediate rudder control signal is within the responsedeadband range, or limits. Thus, as is apparent from the abovedescription of the response deadband circuit 156, the modified ruddercontrol signal of line 160 is set equal to the integral term signal ofline 92. In other words, this condition corresponds to switch 168 ofresponse deadband circuit 156 (FIG. 4) being in position 168b. Alsobefore time t₁, the modified rudder control signal is within the noisedeadband limits. From the above description of the noise deadbandcircuit 158 prior to such time t₁, the rudder control signal of line 54is held constant, as shown. In other words, this condition correspondsto switch 178 (FIG. 4) of noise deadbandcircuit 158 being in position178b.

Consider next the time between t₁ and t₂. The intermediate ruddercontrolsignal is still within the response deadband limits, and thus, themodified rudder control signal is still provided by the integral termsignal. However, at time t₁, the modified rudder control signalintersects the lower noise deadband limit. As is apparent from the abovedescription of noise deadband circuit 158 (FIG. 4) and process steps250-256 (FIG. 6), after time t₁, the rudder control signal is set equalto the modified rudder control signal minus the value of the noisedeadband threshold signal. Moreover, after time t₁, the noise deadbandrange is scaled down, as shown to center such range around the ruddercontrol signal. At time t₂, the modified rudder control signalhasreached a minimum and thus, the noise deadband range is held constantthereafter.

The time period between t₂ and t₃ is like that before time t₁. In otherwords, the intermediate rudder control signal is within the responsedeadband limits, and thus the modified rudder control signal is equal tothe integral term signal. Moreover, the modified rudder control signalis within the noise deadband limits and thus, the rudder control signalis held constant, as shown.

Consider next the intermediate rudder control signal at time t₃ whensuch signal exceeds the upper response deadband limit. Thereafter, themodified rudder control signal is set equal to the intermediate ruddercontrol signal (i.e. switch 168 of FIG. 4 is in position 168a). Also attime t₃, the modified rudder control signal exceeds the upper noisedeadband limit and thus, the rudder control signal is provided by themodified rudder control signal minus the value of the noise deadbandthreshold signal. Note that the modified rudder control signal willcontinue to be provided by the intermediate rudder control signal untilthe intermediate rudder control signal crosses over, or intersects, theintegral term signal and is within the response deadband limits (i.e. asoccurs at time t₄). Thus, at time t₄, the modified rudder control signalis again provided by the integral term signal of line 92 and theresponse deadband limits are reestablished, as shown. Moreover, themodified rudder control signal has reached a minimum and thus, the noisedeadband range is thereafter held constant.

Having described preferred embodiments of the invention, it should nowbecome evident to one of skill in the art that other embodimentsincorporating its concepts may be used. For example, note that whereverthe above description has referred to a rudder of a marine vessel, anysuitable steering mechanism may be substituted therefor. For example, itmay be desirable to apply the circuitry and description provided aboveto an airplane in which the steering mechanism is the airplane's rudder.It is felt, therefore, that this invention should not be restricted tothe disclosed embodiments, but rather should be limited only by thespirit andscope of the appended claims.

What is claimed is:
 1. An autopilot adapted for use on a vessel having asteering mechanism, a source of a signal corresponding to the actualspeed of the vessel, and a source of a signal corresponding to theactual heading of the vessel, said autopilot comprising:means responsiveto said actual speed signal for providing a velocity scheduling signalinversely related to actual speed; operator actuable control means forproviding a signal corresponding to a desired course for said vessel;means responsive to said desired course signal and said actual headingsignal for providing an error signal corresponding to the deviationbetween said desired course and said actual heading; means responsive tosaid error signal and said velocity scheduling signal for generating anintermediate rudder control signal; means for generating a deadbandthreshold signal; means for actuating said steering mechanism inresponse to said intermediate rudder control signal when saidintermediate rudder control signal exceeds said deadband thresholdsignal; and said deadband threshold signal generating means comprisingmeans responsive to said velocity scheduling signal for automaticallyadjusting the value of said deadband threshold signal.
 2. The autopilotrecited in claim 1 wherein said actuating means comprises means forcomparing said intermediate rudder control signal to said deadbandthreshold signal.
 3. The autopilot recited in claim 1 wherein saiddeadband threshold signal generating means further comprises:means forgenerating a first predetermined value in response to said error signal;means responsive to the error signal for differentiating said errorsignal; means for generating a second predetermined value in response tosaid differentiating means; and means for selecting the greater of thefirst and second predetermined values to provide said deadband thresholdsignal.
 4. The autopilot recited in claim 3 wherein said deadbandthreshold signal generating means further comprises:means responsive tosaid velocity scheduling signal for determining a threshold limit value;and means for selecting the greater of the first predetermined value,the second predetermined value, and the threshold limit value.
 5. Theautopilot recited in claim 4 wherein said vessel is a marine vessel andsaid steering mechanism is a rudder.
 6. A marine autopilot for use on avessel having a rudder and a source of a signal corresponding to theactual heading of the vessel, said autopilot comprising:operatoractuable control means for providing a signal corresponding to a desiredcourse for said vessel; means fed by said actual heading signal and saiddesired course signal for generating an error signal corresponding tothe deviation between the desired course and the actual heading; meansresponsive to said error signal for generating an intermediate ruddercontrol signal; means for comparing said intermediate rudder controlsignal to a deadband threshold signal; and means for modifying theintermediate rudder control signal in response to said comparison,wherein said modified rudder control signal is related to the integralof the error signal when said intermediate rudder control signal is lessthan said deadband threshold signal.
 7. The marine autopilot recited inclaim 6 further comprising means for generating said deadband thresholdsignal comprising:means for generating a first predetermined value inresponse to said error signal; means responsive to the error signal fordifferentiating said error signal; means for generating a secondpredetermined value in response to said differentiating means; and meansfor selecting the greater of the first and second predetermined valuesto provide said deadband threshold signal.
 8. The marine autopilotrecited in claim 7 wherein said deadband threshold signal generatingmeans further comprises:means responsive to a velocity scheduling signalfor determining a threshold limit value; and means for selecting thegreater of the first predetermined value, the second predeterminedvalue, and the threshold limit value.